SOP: Determination of Volatile Acidity by Steam Distillation

GIFSQ — Global Institute for Food Safety & Quality

Principle

Volatile acids are separated from the fixed acids of the juice by steam distillation and then titrated in the distillate. Fruit juices normally contain very little volatile acidity (only a few milli-equivalents per litre from aliphatic fatty acids); it serves mainly as a quality index — bad fruit may contain acetic acid, while apple juice that has undergone anaerobic fermentation and tomato juice spoiled by flat-sour organisms may contain lactic acid.

Apparatus required

• Steam distillation assembly
• Burette

Reagents

a. Standard 0.1 N sodium hydroxide solution.

b. Phenolphthalein indicator solution.

Procedure

Place the juice in the distillation flask of the steam-distillation assembly

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Steam-distil and collect the distillate containing the volatile acids

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Titrate the distillate with 0.1 N NaOH, using phenolphthalein as indicator, to a faint pink end point

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Note the titre

Calculation

The book gives no calculation expression for volatile acidity — note the titre of 0.1 N NaOH and report it against the volume of juice distilled.

Precautions

• Volatile acidity at its normal low level is only a quality index; interpret elevated values in the context of the product’s history (fermentation, spoilage).

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.